Electrorheological (ER) fluids are dispersions which can rapidly and reversibly vary their apparent viscosity in the presence of an applied electric field. The electrorheological fluids are dispersions of finely divided solids in hydrophobic, electrically non-conducting oils and such fluids have the ability to change their flow characteristics, even to the point of becoming solid, when subjected to a sufficiently strong electrical field. When the field is removed, the fluids revert to their normal liquid state. Electrical DC fields and also AC fields may be used to effect this change. The current passing through the electrorheological fluid is extremely low. Thus, ER fluids are used in applications in which it is desired to control the transmission of forces by low electric power levels such as, for example, clutches, hydraulic valves, shock absorbers, vibrators or systems used for positioning and holding work pieces in position.
U.S. Pat. No. 2,417,508 (issued in 1947 to Willis M. Winslow) disclosed that certain dispersions composed of finely divided solids such as starch, carbon, limestone, gypsum, flour, etc., dispersed in a non-conducting liquid such as a lightweight transformer oil, olive oil or mineral oil, etc., would undergo an increase in flow resistance when an electrical potential difference was applied to the dispersion. This observation has been referred to as the Winslow Effect. Subsequently, investigators demonstrated that the increase in the flow resistance was due not only to an increase in the viscosity, in the Newtonian sense, but also to rheological changes in which the fluid displays a positive yield stress in the presence of an electric field. This relationship is often described using the Bingham plastic model. Yield stress is the amount of stress which must be exceeded before the system moves or yields. The yield stress is a function of electric field and has been reported to be linear or quadratic, depending on fluid composition and the experimental techniques. Measurement of yield stress can be achieved by extrapolation of stress vs. strain curves, sliding plate, controlled stress, or capillary rheometers.
The efficiency of the electrorheological fluid is related to the amount of electrical power required to affect a given change in rheological properties. This is best characterized as the power required for an observed ratio of yield stress under field to the viscosity of the fluid in the absence of a field. From fluid requirements vs. device design considerations, a parameter has been defined as the dimensionless Winslow number, Wn, where; ##EQU1##
Electrorheological fluids which have been described in the literature can be classified into two general categories: water containing, and those which do not require water. Although fluids were known to function without water, for many years, it was believed that ER fluids had to contain small quantities of water which were believed to be principally associated with the dispersed phase to exhibit significant ER properties. However, from an application standpoint, the presence of water generally is undesirable since it may result in corrosion, operating temperature limitations (loss of water at higher temperatures), and significant electrical power consumption.
The present invention is concerned primarily with the preparation of ER fluids which do not contain significant amounts of water and these are hereinafter termed non-aqueous or substantially anhydrous ER fluids. Several patents and publications in the last five years have described non-aqueous ER fluids in which electronically conductive polymers have been utilized as the dispersed particulate phase. U.S. Pat. No. 4,687,589 (Block et al) describes an electrorheological fluid which comprises a liquid continuous phase and, dispersed therein, at least one dispersed phase which is capable of functioning as such when at least the dispersed phase is substantially anhydrous. Preferably, the ER fluid is one which is capable of functioning as such when the fluid itself is substantially anhydrous. The term "anhydrous" in relation to the dispersed phase is defined as the phase obtained after catalyst removal, which is dried under vacuum at 70.degree. C. for three days to a constant weight. In relation to the continuous phase, an anhydrous continuous phase is defined as the phase dried by passage, at an elevated temperature (for example, 70.degree. C.) if required, through an activated alumina column. The dispersed phase described in this patent is an electronic conductor which is a material through which electricity is conducted by means of electrons (or holes) rather than by means of ions. Examples of such phases include semi-conductors, particularly organic semi-conductors. The semi-conductors are defined as materials having an electric conductivity at ambient temperature of from 10.sup.0 to 10.sup.-11 mho/cm, and a positive temperature-conductivity coefficient. The organic semi-conductors described in this patent include materials which comprise an unsaturated fused polycyclic system such as violanthrone B. The aromatic fused polycyclic systems may comprise at least one heteroatom such as nitrogen or oxygen. Phthalocyanine systems such as a metallophthalocyanine systems are particularly preferred. Another class of electronic conductors described in this patent include fused polycyclic systems such as poly(acene-quinone) polymers which may be prepared by condensing at least one substituted or unsubstituted acene such as by phenyl, terphenyl, naphthylene, etc., with at least one substituted or unsubstituted polyacylated aromatic compound such as a substituted or unsubstituted aromatic polycarboxylic acid in the presence of a Lewis acid such as zinc chloride. Schiff's Bases are also described as suitable organic semi-conductors. The Schiff's Bases may be prepared by reacting polyisocyanates with quinones. Aniline black, prepared, for example, by oxidizing aqueous aniline hydrochloride with sodium chlorate is another example of such an organic semi-conductor. The patentees also indicate that other classes of suitable organic semi-conductors are described by H. A. Pohl et al in J. Phys. Chem., 66, (1962) pp. 2085-2095.
More recently, the use of polyaniline suspensions as electrorheological fluids was described by Gow and Zukowski in "The Electrorheological Properties of Polyaniline Suspensions", J. Colloid and Interface Science, Vol. 126, No. 1, April 1990, pp. 175-188. The authors describe the electrorheological properties of suspensions containing polyaniline particles in silicon oil for a range of suspension volume fractions, applied field strengths, shear stresses, and particle dielectric constants. The polyaniline utilized in the studies was synthesized by adding aniline to chilled aqueous hydrochloric acid followed by the addition of an aqueous ammonium peroxydisulfate solution of the same temperature. The initial reactant concentrations were 0.55 mole aniline, 0.1 mole of the ammonium peroxydisulfate and one mole of hydrochloric acid. The polyaniline solids obtained in this manner were divided into four portions, and an aqueous suspension was prepared from each portion and adjusted with sodium hydroxide to a desired pH (i.e., 6,7,8 and 9). The pH of the suspensions was adjusted over a period of days until they remained constant for 24 hours. The hydrophobic powders were then recovered and washed. The authors concluded that suspensions composed of the polyaniline particles in polydimethyl silicone showed a substantial ER response.
In European patent application 394,005 (corresponding to GB 2,230,532) published on Oct. 24, 1990, Block et al describe an electrorheological fluid which consists of silicone oil containing 30 volume percent of dispersed polyaniline. The polyaniline is acidically oxidized aniline prepared by adding aniline (1.2 moles) to a continuously stirred and cooled solution (0.degree.-5.degree. C.) of ammonium persulfate (1.2 moles) in 1500 ml. of 2M hydrochloric acid solution. After drying and grinding, the black polyaniline powder was treated with sodium or ammonium hydroxide in different amounts and for different periods of time. The base-treated polyanilines prepared in this manner were reported to be useful in ER fluids.
European Patent Application 387857 (published Sep. 19, 1990) describes ER fluids comprising an insulated liquid and solid electrolyte particles which may be various inorganic materials or organic polymers. Alkali metal salts of polyethylene oxide complexes and alkali halide-crown ether complexes are given as examples of such polymers.
Japan Hei 3-33194 published Feb. 13, 1991 describes electrorheological fluids containing dispersed organic polymers. The polymers described in this publication are polypyrrole, polydibromothiophene and poly-p-phenylene.
Japan 3139598 published Jun. 13, 1991, describes ER fluids containing organic conductive polymers and electrically insulating oils. The conductive polymer is preferably obtained by subjecting a polymer, obtained by oxidation polymerization, to a dope-removing treatment, or a polymer obtained by treating polyaniline with alkali. Preferably the polymer powder has an insulating layer on its surface. Preferred polymers include polyaniline, polypyrrole, polythiophene and their derivatives.